The present invention relates to methods of preparing porous plaques for use as electrodes in a variety of applications. One principal use is for the porous cathode within a molten carbonate fuel cell. A plaque of the type described herein may also have application as an electrode within an apparatus for recovering tritium produced in the neutron bombardment of lithium compounds.
Molten carbonate fuel cells typically operate at high temperatures of about 900.degree.-1000.degree. K. to convert chemical energy to D.C. electricity. Fuels such as H.sub.2, CO or methanol react with oxidant gases, for instance, air or oxygen including carbon dioxide during this production of electrical energy. Typical reactions are as follows: ##STR1##
It is contemplated that these fuel cells will typically operate in stacks of repeating elements. Each element contains an anode, a cathode and an electrolyte structure separating the two electrodes. In the molten carbonate cell, anode structures typically include porous sintered nickel alloyed with chromium or cobalt for strength. The electrolyte structure can be a porous tile of lithium aluminate filled with a molten carbonate electrolyte. The electrolyte tile should include an appropriate pore structure to permit wetting without flooding of the adjacent electrode. Typical electrolyte tiles and molten carbonate salt electrolytes are presented in U.S. Pat. No. 4,115,632 to Kinoshita et al and in U.S. Pat. No. 4,251,600 to Sim et al.
Early efforts in providing cathodes for molten carbonate fuel cells have involved assembling the cell with a porous nickel plaque as a cathode. On raising the cell to operating temperature and exposing the plaque to oxygen and molten carbonate the nickel cathode plate oxidizes in situ to NiO and incorporates a small percentage e.g. 2-3 atom percent lithium. Nickel oxide (NiO) is a deficient semiconductor (P-type) that exhibits rather poor electrical conductivity. Lithiation, that is the incorporation of lithium within the nickel oxide lattice has been found to provide an enormous enhancement in the P-type conductivity. However, substantial difficulties have arisen in the preparation of nickel oxide cathodes with in situ oxidation and lithiation of the sintered nickel plaque. For example, substantial cathode swelling with accompanying lithium loss from the electrolyte has made it desirable to provide alternate approaches to the preparation of porous cathodes for the molten carbonate fuel cells.
Previous attempts to fabricate sintered nickel oxide plaques for use as cathodes have involved forming sinters of lithiated nickel oxide. Due to the volatility of lithium oxide, only a narrow range of sintering conditions has been found to be appropriate for both retention of lithium within the porous structure and for providing sufficient sintering to impart good mechanical strength and integrity to the cathode structure. Where temperatures much above 1100.degree.-1200.degree. C. are employed, the lithium is driven from the NiO lattice within a short time. Temperatures below about 1000.degree. C. have failed to produce sintered structures of sufficient physical integrity for use within fuel cells.
Efforts to lithiate NiO plaques previously have not been successful due to the procedures adopted. Often the use of elevated temperatures or extended exposures have volatalized lithium from the NiO lattice.